1. Field of the Invention
The present invention relates to methods for annealing a substrate, and particularly to an apparatus and method for annealing a semiconductor substrate so as to manufacture semiconductor devices using a high intensity light source, and a manufacturing method of semiconductor devices using the annealing method.
2. Description of the Related Art
It is possible to achieve improvements in performance of a semiconductor device having a large scale integration (LSI) by increasing integration density, or to put it more plainly, by miniaturization of the elements that build up a semiconductor device. Thus, the degree of on-chip integration of LSIs are increasing larger and larger while miniaturization of elements, such as metal-oxide-semiconductor (MOS) transistors, is being taken to a whole new level. Along with the miniaturization of elements, parasitic resistance and short channel effects on MOS transistors and the like, are increasing. Thus, there is increased importance placed on the formation of low resistance layers and shallow pn junctions.
For forming a shallow pn junction with a thickness of or below twenty nm, impurity ions are implanted in a semiconductor substrate with low acceleration energy so as to establish a shallow projected range. The impurities doped in the semiconductor substrate are activated by annealing, thus forming a shallow impurity diffusion region. In order to decrease layer resistance of an impurity diffusion region, it is necessary to perform activation annealing of the impurities at a high temperature.
However, the diffusion coefficients of p-type impurity such as boron (B), and n-type impurity such as phosphorus (P) or arsenic (As), in the crystal of the silicon (Si) substrate, are large. In the processing time needed to perform rapid thermal annealing (RTA) using current halogen lamps, impurities diffuse to both the interior and exterior of a semiconductor substrate. As a result, it is impossible to form a shallow impurity diffusion region having a high concentration of impurities on the semiconductor substrate. Also, it becomes impossible to activate a high concentration of impurities if the temperature of the RTA process is decreased in order to control the diffusion of the impurities. Because of such difficulties, it is difficult to form a shallow impurity diffusion region having low resistance and a high concentration of activated impurities.
Recently, a pulse light annealing method by the use of a pulse light source, such as a flash lamp and a YAG laser, which can instantly supply the energy essential to impurity activation, is being tested as a solution to the RTA problem (refer to U.S. Pat. No. 4,151,008). A xenon (Xe) flash lamp has a quartz glass tube filled with Xe gas, in which electrical charges stored in capacitors and the like, are instantaneously discharged. As a result, it is possible to emit a high intensity white light within a range of several hundred μs to several hundred ms. It is possible to attain the heat energy required for impurity activation in the instantaneous heating of a semiconductor substrate absorbing flash lamplight. Therefore, it is possible to activate a high concentration of impurities while leaving the concentration profile of the impurities, implanted into the semiconductor substrate, virtually unchanged.
However, in using flash lamp annealing, irradiation energy above 20 J/cm2 is essential to ensure a sufficiently uniform activation of impurities at a high concentration, which would lead to a sudden temperature increase on the semiconductor substrate. As a result, there occurs a temperature difference in between a top surface and a bottom surface of the semiconductor substrate, which raises the amount of thermal stress in the interior of the semiconductor substrate. Especially, a total amount of thermal stress generated in the semiconductor substrate having a larger area increases. Thermal stress causes crystal defects such as dislocations and slip. The presence of crystal defects makes it easy for damage to occur in the semiconductor substrate, leading to decreases in a manufacturing yield. Thus, it is difficult to perform annealing with flash lamp annealing processes while suppressing the generation of damage generated in the semiconductor substrate.